Compounding interest in nanotubes

Jon Lawson

To make non-marking tyres for dangerous environments, conductive fillers need to be added at the compounding stage. Nanotubes excel here, as Anastasia Zirka explains

There are a number of standards relating to regulations for tyres and their electrical properties. As is often the case with standards, their titles can be bit of a mouthful, such as the International Standard ISO 16392 Electrical resistance — Test methods to measure the electrical resistance of tyres on a test rig, or ASTM F 1971 – Standard test method for electrical resistance of tyres under load on the test bench.

These requirements regulate and ensure Electrostatic discharge (ESD) protection to remove static electricity from the surface of a tyre when it occurs at high speed and to prevent its accumulation in the long run.

In some application areas, normal tyres that are required to be anti-static are considered to be capable of safely dissipating electrical charge if the resistance measured does not exceed 1010Ω.

But there are a number of specific cases, such as when the tyre is used in potentially explosive areas, that are strictly regulated by the ATEX Products Directive 2014/34/EU and EN 1755 – Industrial trucks – safety requirements and verification for example, which include supplementary requirements for operation in potentially explosive atmospheres. According to EN 1755, the outer material of castors and wheels should have a maximum surface resistance of 109Ω.

It is a well-known fact that most tyre formulations contain carbon black and can easily achieve the requirements for being anti-static, but this is not the case with non-marking solid tyres. Using graphene nanotubes (also referred to single wall carbon nanotubes, SWCNTs) is an alternative solution to provide the required conductive properties as well as maintain colour and the basic formulation properties of non-marking solid tyres.

It has been acknowledged by both scientists and manufacturer communities that graphene nanotubes are one of the most advanced additives for improving almost all materials used in our daily life. When applied in rubber compounds, they enhance mechanical performance while also imparting conductivity. The key to achieving this previously unobtainable combination of properties is graphene nanotubes’ extremely low working dosage.

Nanofillers - consistent conductivity versus mechanical performance

Various types of inorganic nanofillers have proven to be an efficient way to enhance the mechanical and electrical properties of rubbers. Carbon black and silica are widely used in commercially produced rubbers, including for tyres.

It is known that the shape of the nanofiller affects the critical concentration required to create a connected network of filler (called the percolation threshold); the higher the aspect ratio of the nanoparticle, the lower the percolation threshold. That is why using 2D and 1D carbon nanofillers (i.e. graphene sheets and carbon nanotubes) in rubbers has attracted the attention of numerous research groups in academic institutions and in industry. Graphene nanotubes are characterised by the smallest possible diameter of less than 2nm, and are of particular interest.

One fairly new conductive additive that has recently entered the polymer market is multi-wall carbon nanotubes (MWCNTs). However, MWCNTs are not able to fully meet manufacturers’ needs for stable levels of conductivity and easier compounding, or offer suitable mechanical properties and softness requirements. To be effective in the host material, the required concentration of thick and short MWCNTs is tens or even hundreds of times higher than that of long and thin SWCNTs.

Despite the similarity in names, multi-wall carbon nanotubes and single wall carbon nanotubes – two allotropes of carbon – have hardly anything in common, and thus they impart a number of very different properties.

While MWCNTs feature many tubes of decreasing diameter that are coiled inside each other, SWCNTs can be thought of as an extremely thin rolled-up sheet of graphene, and thus they are widely referred to as graphene nanotubes. Being an excellent conductor similar to copper and 100 times stronger than steel, graphene nanotubes gain traction starting from a loading of just 0.01%. Even such an ultra-low concentration is enough to enable conductivity without a negative effect on the material’s mechanical properties or viscosity.

Currently, graphene nanotubes are the most innovative additive that facilitates efficiency, durability and affordability in solid tyres, particularly for coloured or non-marking applications where anti-static or conductive properties are required for tyres to dissipate the static charge.

To impart conductive properties to white filler-based rubbers, conductive fillers need to be added during the compounding process. There are not many additives which can reach the desired level of electrical conductivity without affecting the colour. Depending on the particular application, it could be copper powder, silver- and nickel-coated graphite, copper or aluminium – while these do provide the required conductive properties, adding them also leads to a number of negative trade-offs in flexibility, elasticity and high price.

For conventional polymer-based additives which are more commonly used due to their low price, the high working dosages required result in a number of drawbacks in the rubber’s mechanical properties and a reduced life cycle of the final products. These shortcomings severely restrict the potential range of non-marking solid tyre applications. Furthermore, conventional polymer-based additives are normally humidity-dependent and subject to migration, so the rubber’s properties are not stable because of this migration to the surface and release during the tyre’s life cycle.

Historically, there have been several technical barriers that have prevented the mass application of graphene nanotubes in various industries, including elastomers. The main challenge was related to the difficulty in achieving uniform and homogeneous dispersion in the material matrix and in preventing further agglomeration. However, recent materials technology advances have enabled easy-handling nanotube concentrates that are compatible with industry-standard formulations and manufacturing processes, without requiring any specific changes or additional tools.